Unless I'm wrong in release 1.3.1, FigureCanvasGTK3Cairo does not define
blit(), neither in parents FigureCanvasCairo nor in FigureCanvasGTK3.
However from FigureCanvasBase it inherits of supports_blit = True.
Therefore when using widgets such as SpanSelector, the test "if
self.useblit" is true and cause a call to copy_from_bbox which is
undefined (actually copy_from_bbox is defined in FigureCanvasAgg).
Setting supports_blit = False on Canvas instantiation seems to fix the
issue for now. Is my understanding correct ?